1. Field of the Invention
The present invention relates to a technique to correct fluctuations in density that occur on the periphery of an edge of multivalued data obtained by performing halftone processing on digital image data.
2. Description of the Related Art
In the case where digital image data each color of which is represented by 8 bits or 16 bits is printed, halftone processing to convert the digital image data into multivalued data having the number of tone levels that the printer can print is performed while representing the original gradation in a pseudo manner. In the case where the printer is a laser beam printer that forms an image by using an electrophotographic process, the time during which laser light is emitted in the exposure process is determined by performing PWM control in accordance with the multivalued data on which the halftone processing has been performed.
In the case where an image is formed by using the electrophotographic process, it is known that fluctuations in density occur on the periphery of the edge of the image due to a phenomenon (called an edge effect) in which the magnetic toner is developed in the state of being concentrated at the edge portion of a character or the like because electric fields concentrate on the edge of an electrostatic latent image located on the photoconductor. The degree in which the edge effect appears differs also depending on the characteristics of each process forming the electrophotographic process, in addition to the image factors, such as the direction, length, and density, of the edge. In particular, the edge effect will be remarkable on the periphery of an edge at which the density becomes lower in the sub scanning direction and the lower of density values represents the background color (hereinafter, called a bottom-end edge).
In order to deal with such a phenomenon, in Japanese Patent Laid-Open No. 2007-272153, a technique has been proposed that performs an arithmetic operation on the value of the pixel of interest on image data based on the value of a pixel located on the downstream side in the sub scanning direction of the pixel of interest, and corrects the image data in accordance with the arithmetic operation results.
Further, in Japanese Patent Laid-Open No. H11-028839 (1999), a technique has been proposed, in which an edge pixel that changes in the sub scanning direction is extracted, the position and pixel value of the edge pixel on the printing medium are determined, and correction is performed by dividing the values of pixels having intermediate pixel values of the input image data into a first area far from the edge pixel and a second area on the edge pixel side.
In Japanese Patent Laid-Open No. 2007-272153 described above, the image data is temporarily held in the image memory from an external storage device, the arithmetic operation is performed on the data value of the original image data within the image memory and the data value of virtual image data that has been translated in the sub scanning direction, and the image data is corrected in accordance with the arithmetic operation results. In the case of this technique, it is necessary to hold the image data to be printed and output at least by an amount corresponding to the number of lines of translation in the image memory, and therefore, the increase in the cost resulting from the correction processing is not small.
Further, in Japanese Patent Laid-Open No. H11-028839 (1999), the edge pixel (pixel whose pixel value changes from the intermediate pixel value into the background pixel value in the sub scanning direction) is extracted and in order to determine the position of the bottom-end edge and the number of intermediate pixel values down to the bottom-end edge, the pixel values are input in the sub scanning direction. Because of this, it is necessary to perform pre-processing to change the pixel data columns arranged in the main scanning direction into those arranged in the sub scanning direction by performing 90° C. rotation after generating print data corresponding to one page, or to generate data rotated by 90° C. at the time of generating print data corresponding to one page. However, it is necessary to rotate the data by 90° C. before outputting the data to the print engine. In either case, a buffer for 90° C. rotation is necessary, and therefore, the cost will be pushed up considerably as a result.